The present invention relates to reduction of noise in an avalanche photodiode (hereinafter referred to as the "APD") for use in optical communication equipments or the like.
In the APD, carriers excited by incident light are accelerated and multiplied by a high intensity electric field emanating from a pn junction but, due to fluctuation in this course of amplification, excess multiplication noise is produced. It is known that, letting the inonization coefficients of electrons and holes be represented by .alpha. and .beta., respectively, and assuming that the ratio therebetween is k=.alpha./.beta. or .beta./.alpha., an excess noise factor decreases in proportion to the ratio k. Accordingly, in order to obtain an APD of low noise, it is necessary that the carriers of the larger ionization coefficient be injected into a material of a large ionization coefficient ratio k for multiplication.
At present, APDs using silicon crystals are widely employed as photo detectors in optical communications in the vicinity of a 0.8 .mu.m wavelength region which utilize GaAs-AlGaAs system light emitting devices, but this is because silicon has as large an ionization coefficient ratio as k=.alpha./.beta..perspectiveto.50 and is suitable for use in the low-noise APD. However, the Si-APD cannot be used in a 1.0 to 1.7 .mu.m wavelength range in which transmission loss of a silica fiber employed in optical communications is low. Heretofore, Ge-APD using germanium crystals has been employed as a photo detector having effective sensitivity to a 1 .mu.m wavelength region, but since the ionization coefficient ratio of germanium is k=.alpha./.beta..perspectiveto.1, the Ge-APD suffers from large excess noise and cannot be regarded as an optimal photo detector. On the other hand, there are also under development APDs which use the III-V compound semiconductor crystals, such as InGaAsP, AlGaAsSb and so forth, and have effective photosensitivity to the 1 .mu.m region. However, there is a limit to the decrease of noise in the APDs using such semiconductor materials in prior arts.